This invention relates to a novel and useful drug-delivery device for releasing a pharmaceutically acceptable drug at a controlled and continuous rate for a prolonged period of time to produce a physiological or pharmacological effect. The drug delivery device is comprised of a drug release rate controlling polymeric material surrounding at least a part of the drug or the material contains the drug therein. The material is permeable to the passage of drug by diffusion.
Often, in therapeutic, medical and veterinary programs, it is desirable and important to provide for the slow release of a drug to the body at a controlled rate over a prolonged period of time. In many therapeutic programs such a rate of release should be constant or have a zero order time dependence, that is, the rate of release is independent of time. For example, in contraception, it has been found that fertility can be regulated by release of progesterone to the uterus from a drug-delivery device inserted in the uterine lumen; with release of progesterone at a constant rate for a prolonged period, a convenient long term birth control procedure is provided. Similarly, glaucoma can be controlled by release of pilocarpine from a thin film drug-delivery device inserted beneath the lower eyelid. Here too, the active beneficial agent should be released at a constant rate.
Different approaches have heretofore been tried to obtain a drug-delivery device for releasing drug at a controlled rate. One approach, which has received great attention, is to mix the drug with a carrier material that is gradually broken down by body fluids, the drug being released as the carrier disintegrates. Numerous carriers have been used in such systems including waxes, oils, fats, soluble polymers and the like. While some of these systems have provided for a delayed release of the drug, constant release rate has not been obtained. One reason for this is that as the carrier disintegrates the surface area of the dosage unit decreases, concomitantly exposing increasingly smaller quantities of the carrier to the surrounding body fluids. This inherently results in a decline in the release rate over time. Moreover, there has been little success in gaining control over drug release rate by this approach.
Another approach has been to enclose the drug within a capsule having polymeric walls through which the drug can pass by diffusion. An approach of this kind is set forth in U.S. Pat. No. 3,279,996. These devices too have inherent difficulties. These prior systems have generally been based on the use of silicone rubber polymers, especially polydimethylsiloxanes, as the diffusion control membrane. In large part, these silicone polymers were selected because of their high permeability to some important drug molecules. But it has now been found that this high permeability can be a significant disadvantage which defeats the primary objective of an acceptable and useful drug-delivery device. Thus, with many important drug molecules, such as progesterone, the diffusion rate through polydimethylsiloxane membranes is very great, often greater than the rate of clearance of the diffused drug from the outer surface of the capsule. In many instances, this results in the rate limiting step being clearance of the drug from the exterior of the capsule, rather than diffusion through the capsule wall. Clearance rate within the body is difficult to control, as it is subject to frequent changes and, when it is the rate-limiting step, the objective of providing a drug-delivery device which releases drug at a constant rate over time cannot be obtained. Also, silicone rubbers have a tendency to absorb lipoidal materials from the body. When this effect occurs in vivo, the nature of the membrane changes, altering the release rate. Still another problem with the silicone rubber dosage forms is the difficulty encountered in fabrication, attributable largely to the silicone rubber itself, that is, it is not thermoplastic and accordingly it does not lend itself to the manufacture of heat sealed drug delivery devices. While the above mentioned polymer, silicone, has been found to have too high a permeability to many therapeutic drugs to lend itself to successful use as a drug release rate controlling barrier, in other instances, polymers such as poly(ethylene), which is thermoplastic and has a high degree of crystallinity, has also been found to have too high a melting point that can adversely affect the drug during fabrication of the device, and because of its crystallinity has too low a permeability to drug to be successfully used as a drug release rate barrier.
In unrelated, non-therapeutic fields, various other polymeric materials have been used for releasing active substances. For example, a closed container whose walls are comprised of ethylene vinyl acetate copolymer is described in U.S. Pat. No. 3,310,235 as seemingly suitable for releasing volatile, organic and toxic phosphorous biocides by the process of physical evaporation. With these containers, all biocide release is achieved by evaporation from the surface, and if the ingredient is not sufficiently volatile at the use temperature the container has no apparent practical value. Evaporation is preferably achieved by using a woven cloth which acts as an evaporation surface. However, release rate by evaporation is difficult to regulate and virtually impossible to control as it is subjected to uncontrollable environmental conditions, the vapor pressure of the substance, and the degree of saturation of the volatile substance in the environment. The type of clearance inherently defeats the basic purpose of providing a drug delivery device which releases a drug at a controlled rate for a prolonged period of time such as by the process of diffusion.
It is also known to the art to use polymeric materials such as ethylene-vinyl acetate copolymer in other forms and for different purposes. For example, in Great Britain Pat. No. 1,126,849 there is disclosed a cellular polymer in which open cells are purposely formed for the movement of gases into these pores. These cellular polymers are disclosed to have non-medical uses and generally these polymers do not lend themselves to use as diffusional drug devices. In U.S. Pat. No. 3,400,011 polymeric materials are mixed with waxes and used for coating ingredients that are subsequently released by the osmotic movement of external fluids into the coating causing it to rupture and release the surrounded ingredient. In U.S. Pat. No. 3,618,604 there is disclosed an ocular drug delivery device that represents a substantial improvement over previously proposed drug devices and which devices can be successfully used for their intended purpose in the management of ocular medicine. But, the use of some of the materials set forth therein, for example, partially hydrolyzed polyvinylacetate because of its gel like properties, has led to manufacturing difficulties and also has not given the desired drug release rates in many instances. Other incidental and non-therapeutic uses for vinyl acetate copolymers are disclosed in French Pat. No. 1,489,490 as a thickner and in French Pat. No. 1,505,267 as a non-diffusional formless base for chewing gum.